Formula: Be3Al2(Si6O18)
Reflected/transmitted colour: usually colourless
Pleochroism: in thick sections weakly pleochroic, e.g. emerald may show w yellowish green, ε sea-green
Optical isotropy/anisotropy: uniaxial
Optical sign: -
Refractive indices: w = 1.565-1.599, ε = 1.569-1.610
Birefringence δ: 0.004-0.009
2V angle:
Dispersion: weak
Optical orientation:
Optical axial plane:
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